Heretofore, the mark sheet designating the address and scanning line resolution of a document has been scanned by means of the optical detecting device of a facsimile apparatus before a document is scanned or read by the facsimile apparatus. A plurality of mark writing regions, or user mark regions, are arranged on a mark sheet in rows and columns, and the user is requested to mark regions by black color. Said optical detecting device is, for example, a CCD (charge coupled device) array in which a plurality of optical sensor elements are arranged in line. The mark sheet is moved to traverse the optical detecting device so that the mark rows are sequentially scanned. The mark sheet is illuminated with a light source, and the marks are detected by directing the reflected light to the optical detecting device through lens.
When the mark sheet travels through the optical sensor, the mark sheet undesirably skews and some or all mark regions in a mark row are out of the optical sensor, resulting in decreased reading accuracy. In order to solve this problem, a published unexamined Japanese patent application No. 50-8411 (1975) discloses a solution wherein two special marks, i.e., a reference mark and a skew detecting mark, are provided on the left and right ends of the uppermost, or the first mark row of the mark sheet, and the scanning lines are switched over by detecting the skew of the mark sheet. However, the method of published unexamined patent application No. 50-8411 cannot be applied to the reader to scan only mark regions by giving addresses to optical sensor elements linearly arrayed in order from an end, and designating the addresses corresponding to the mark regions, or the scan window addresses as in the present invention. The width of the mark regions in the scanning line direction is normally specified to be narrow (e.g. 2 mm), and the scan window addresses must be aligned correctly against the mark regions during reading the mark sheet. However, if the skew of the mark sheet changes while the mark sheet travels, the above alignment is lost and correct mark reading is not possible. By the above method of the published unexamined patent application No. 50-8411, the alignment problem is not solved because the skew of the mark sheet is detected only at the leading portion of the mark sheet and the scanning lines are selected equally for all the mark rows.
The above misalignment is also caused by the enlargement or reduction of the sheet image due to the deviation of the lens fixing device or the variation of the optical characteristics of the lens itself, the use of the mark sheet having dimensionally misprinted portions, or the change in the skew of the mark sheet during reading the mark sheet, and the marks cannot be read accurately. The present invention solves such problems.